The present invention relates in general to a method and related apparatus for cooling a heat source, and in particular to a method which employs a circulating liquid metal coolant composition as a heat dissipation medium within a closed-loop containment vessel of a cooling system. The interior surface of the vessel is covered with a protective coating such as an oxide layer to prevent an untoward reaction between the vessel and the liquid metal composition.
Traditional heat sources that require proactive heat removal include process systems such as those exemplified by internal-combustion engines, gasoline-driven and coal-driven electricity generators, nuclear reactors, accelerator-driven radioactive waste transmutators, spalation sources used in nuclear accelerators, and the like. Efficient cooling systems have been developed that utilize liquid metal compositions as heat absorbers, and such liquid metal systems are usually found in association with nuclear reactors and related equipment that generate significant heat during operation. The desirability of liquid metal compositions for heat removal is attributed to liquid metal properties that include high thermal conductivity, thermal stability, low neutron capture cross section (resulting in relatively uniform power distributions), self shielding from reactor gamma-rays, high boiling points (enabling in low-pressure operation at high temperatures), and high capacities for heat absorption, storage, and dissipation.
Liquid metal cooling systems operate in much the same manner as do the aqueous-coolant cooling systems for conventional internal combustion engines found in vehicles. Thus, in conventional liquid metal cooling systems, the liquid metal is confined in a closed-loop system which includes a heat source portion and a heat exchanger portion. Operationally, the heat source portion comes into thermal communication with a heat source (e.g. a nuclear reactor) and heat therefrom transfers into the liquid metal composition as it travels through the heat source portion of the closed-loop. As a result, the temperature of the liquid metal composition increases as the composition passes through the heat source portion. After absorption of heat, the liquid metal composition continues its travel within the closed-loop for ultimate arrival at the heat exchanger portion where the absorbed heat is dissipated and the composition continues in the closed-loop for return to the heat source portion as the circuit repeats.
The closed-loop containment vessel described above is generally constructed from an alloy pipe, with steel usually being the material of choice because of its physical properties which primarily include compatibility with high heat coupled with favorable economic considerations. Beyond these considerations, however, is the need for compatibility between the containment vessel and the liquid metal composition therein. In this regard, and unfortunately, molten sodium, lithium, lead, bismuth and their respective alloys readily corrode steel and steel alloys. As is generally recognized, corrosion is the process by which a molten metal cooling composition destroys another metal (such as the containment vessel of a closed-loop system) and, for this reason, the suitability of many metals for cooling purposes is severely limited.
Lead and lead alloys are of particular interest in liquid metal cooling systems. While lead and lead alloys in liquid metal cooling systems offer several advantages, lead compositions are particularly aggressive to most metal components in these systems. The aggressive nature of liquid lead compositions has resulted in trial systems manufactured from exotic materials supposedly immune to attack, but which experimentally show that lead-based problems continue to exist. Likewise, prior approaches for solving lead incompatibility have included the provision of additives and inhibitors, diffusion coatings, and plasma deposition. Thus, additives and inhibitors such as uranium, magnesium, zirconium, titanium, tellurium, thorium, calcium, chromium, and tungsten were studied for corrosion control properties, with reductions in corrosion rates being accomplished by zirconium, tungsten, and chromium. Regarding the application of diffusion coatings, U.S. Pat. No. 4,242,420 to Rausch et al. teaches application of a diffusion coating on a ferrous substrate by introducing a molten alloy bath basically consisting of lead and chromium to thereby coat chromium on iron. The resulting coating, however, was rough and porous. Finally, plasma deposition of molybdenum, zirconium, or carbide salts on the surface of a metal has been performed to provide a protective layer. However, all of the above-described methods of corrosion control suffer from erratic adherence of the protective coating and non-uniformity of the protective layer, conditions that are unacceptable in many applications.
Another approach that has been employed for the inhibition of corrosion is the provision of an oxide layer on the affected surface. Such oxide layers can be produced by oxygen-bearing gases introduced into the molten metal cooling composition, but the quantity of oxygen, and therefore oxidation, is critical to controlling the formation of the oxide layer. Conventional methods for monitoring oxygen levels in molten metal cooling compositions use zirconia probes originally developed for the measurement of oxygen in liquid-sodium cooling systems. Reliability of these zirconia probes in a molten metal cooling composition (especially lead) is known to be problematic and thus can result in the continuous formation of an oxide layer which will eventually shut down the flow path for coolant. Furthermore, because prior techniques do not provide for the reversal of excess oxidation, such coolant flow shutdown can cause catastrophic equipment damage.